Interaction of T4 AsiA with its Target Sites in the RNA Polymerase σ 70 Subunit Leads to Distinct and Opposite Effects on Transcription

Abstract

Bacteriophage T4 AsiA is a homodimeric protein that orchestrates a switch from the host and early viral transcription to middle viral transcription by binding to the σ 70 subunit of Escherichia coli RNA polymerase holoenzyme (Eσ 70) and preventing promoter complex formation on most E. coli and early T4 promoters. In addition, Eσ 70AsiA, but not Eσ 70, is a substrate of transcription activation by T4-encoded DNA-binding protein MotA, a co-activator of transcription from middle viral promoters. The molecular determinants of σ 70–AsiA interaction necessary for transcription inhibition reside in the σ 70 conserved region 4.2, which recognizes the −35 promoter consensus element. The molecular determinants of σ 70–AsiA interaction necessary for MotA-dependent transcription activation have not been identified. Here, we show that in the absence of σ 70 region 4.2, AsiA interacts with σ 70 conserved region 4.1 and activates transcription in a MotA-independent manner. Further, we show that the AsiA dimer must dissociate to interact with either region 4.2 or region 4.1 of σ 70. We propose that MotA may co-activate transcription by restricting AsiA binding to σ 70 region 4.1.